A conventional focusing method of a zoom lens is a method of feeding the first lens unit, so called a front lens feeding method. It is because this method has the advantage that it enables focusing by an identical feed amount for an object at an identical object distance throughout the entire variable magnification range and is realized in a simple structure. (For example, reference is made to Patent Document 1).
Patent Document 1: Japanese Patent Application Laid-open No. 11-258504